Magnetorheological (MR) devices of the “rotary-acting” or “linear-acting” variety such as linear dampers, rotary brakes and rotary clutches employ magnetorheological materials as dry particles or particles dispersed in fluids occupying the working gap within the device. The particles are comprised of magneto-soft particles. The higher the applied magnetic field strength, the higher the damping or resistive force or torque needed to overcome the particle structure aligned within the field.
MR devices are disclosed in U.S. Pat. No. 5,816,372 entitled “Magnetorheological Fluid Devices And Process Of Controlling Force In Exercise Equipment Utilizing Same”; U.S. Pat. No. 5,711,746 entitled “Portable Controllable Fluid Rehabilitation Devices”; U.S. Pat. No. 5,842,547 entitled “Controllable Brake”; U.S. Pat. No. 5,878,871 entitled “Controllable Vibration Apparatus” and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention, and incorporated herein by reference.
The present invention is directed to dampers that include a housing or chamber that contains the magnetically controllable fluid disclosed hereinbelow, with a movable member, a piston or rotor, mounted for movement through the fluid in the housing. The housing and the movable member both include a magnetically permeable pole piece. A magnetic field generator produces a magnetic field across both pole pieces for directing the magnetic flux to desired regions of the controllable fluid. Such devices require precisely toleranced components, expensive seals, expensive bearings, and a relatively small volume of magnetically controllable fluid. MR devices as currently designed are comparatively expensive to manufacture. There is a continuing need for reducing the cost of controllable MR devices for providing variable forces and/or torques.
Conventional MR fluids containing magnetically active fine particles generally on the order of 1-100 μm average diameter employ conventional iron particles manufactured by the carbonyl process, whereby particles are grown by precipitation of pentacarbonyl salts. The cost of carbonyl powders are notoriously high. Magnetorheological fluids have been manufactured that employ magnetically active particles manufactured by an atomization method, which is a reductive process of dividing a molten metal stream into small particles. The molten metal stream is delivered into a high pressure, high velocity stream and divided by high shear and turbulence (hereinafter collectively referred to as “atomized particles”).
Due to performance and cost concerns, suitable replacement for expensive carbonyl iron by atomized particles has not heretofore been a straightforward substitution. In conventional practice, atomized particles of a single process stream have been sieved to exclude a significant fraction of 10-20% of particles larger than 74 μm. In other examples, even larger fraction of 20-30+% of a single process yield of atomized particles greater than 45 μm size must be excluded. Removal of such unusable volume fractions representing yields of even 90% and below are now considered uneconomical.
Attempts have been made to blend atomized particles with carbonyl iron particles to achieve a suitable particle size distribution for use in MR dry powders and MR fluids. Heretofore, attempts to provide 100% of conventional atomized particles passing through a 74 μm sieve approaching toward a Gaussian distribution have been achieved by blending particles from more than one process stream. One example is a blend of carbonyl iron with atomized particles. U.S. Pat. No. 6,027,664 (Lord Corporation) teaches blends of a first population having an average particle diameter 3 to 15 times larger than the second population. The smaller average sized particles are carbonyl iron and larger size particles are atomized iron. Such mixtures suffer economically from yield losses carried over from classification or sieving, and costs associated with making blends per se.
The suitability of any particulate metals for use in MR fluids is in one respect determined by analyzing the deviation from a Gaussian distribution, and can be illustrated by a regression analysis. Mixtures of two different populations heretofore taught in the art have provided a degree of conformity to a Gaussian distribution that approaches the distribution of carbonyl iron-based particles. For example, a 50:50 wt. mixture of carbonyl iron and water-atomized particles available as of the filing date in the '664 patent exhibit a log normal size distribution, R2, of 0.82. Although technically feasible, the particle blends heretofore available suffer from the same economic drawbacks. A need therefore exists for MR devices utilizing controllable powder or MR fluids utilizing particles of a lower cost single process stream to overcome the economic drawbacks. It would be advantageous to provide a MR fluid containing a particle component derived from a single process yield population of magnetically responsive particles exhibiting a suitable size population, and size distribution for improving economic factors in controllable devices.